There are many ways to validate or authenticate a value document, from simple to complex. Some methods involve visible, also referred to as overt, features on or incorporated into a document, such as a hologram on a credit card, an embossed image or watermark on a bank note, a security foil, a security ribbon, colored threads or colored fibers within a bank note, or a floating and/or sinking image on a passport. While these features are easy to detect with the human eye and generally do not require equipment for authentication, these overt features are easily identified by a would-be forger and/or counterfeiter. As such, hidden, also referred to as covert, features may be incorporated in value documents, either in lieu of or in addition to overt features. Covert features can include invisible fluorescent fibers, chemically sensitive stains, fluorescent pigments or dyes that are incorporated into the substrate of the value document. Covert features can also be included in the ink that is printed onto the substrate of the value document or within the resin used to make films that are used in laminated value documents. Since covert features are not detectable by the human eye, detectors configured to detect these covert features are needed to authenticate the value document.
Some covert features incorporate taggants that absorb radiation from a light source and emit detectable radiation having properties, such as wavelength and decay time, which can be used to determine whether the value document incorporating the feature is authentic. For example, some covert taggants use rare earth active ions that have been incorporated into oxide crystal lattices. Most oxide crystal lattices are essentially closed packed structures of oxygen ions where metallic ions may be connected to oxygen atoms of the crystal lattice resulting in symmetries of the crystal fields. Oxide crystals typically result in rapid decay of absorbed radiation to a storage energy level followed by decay to lower energy levels. When such covert taggants are illuminated by a light source, they tend to have a peak in their intensity of emitted radiation that corresponds to the point in time when the light source is shut off and thereafter, the emitted radiation exhibits rapid emission decay.
FIG. 1 is a graph 100 illustrating intensities (in arbitrary units, AU) of an excitation signal 102 and an emission signal 104 with respect to time for one example of a prior art system in which a value document including a covert taggant is illuminated by an LED light source for 10 milliseconds. As illustrated, the intensity of the radiation emitted from the taggant builds during the time that the light source is on, and then decays rapidly once the light source is shut off, decreasing exponentially, as expected, for many existing luminescent covert taggants. The exponential decay constant is a function of the specific taggant used, the host lattice material, and the doping amounts of substitute ions, and is defined as the time required for the emission intensity to decay to the 1/e value. This “1/e value” is referred to as the lifetime “Tau” (τ).